Instant invention relates to a composition for selectively etching films of aluminum metal or alloys thereof in the manufacture of microelectronic devices, printed circuits, photolithographic plates and the like. Another aspect of the invention relates to the method of selectively etching unprotected areas of an aluminum film, deposited on the surface of a substrate, with exceptionally fine line dilineation, definition and optical resolution such as is required in the manufacture of relatively small, high precision devices, semi-conductors and integrated microelectronic and printed circuitry.
Typically, the fabrication of devices for microelectronic applications involves etching of a circuit pattern through a thin layer of unprotected aluminum film, or an alloy thereof containing a predominant proportion of aluminum metal. The purpose of the etchant is to selectively remove exposed areas of metal film without injury to other resist coated areas of the film so that on completion of the etching operation and removal of the photoresist coating, there remains unaltered metal film in predescribed areas on the substrate to serve as connecting pins or interconnections in a semi-conductor or circuit device. Generally, the procedure involves coating a substrate such as glass, ceramic, silica, silicon or a plastic or fiber substrate, such as a sheet, with a film of the metal deposited thereon by electroplating, sputtering, evaporation, or lamination to provide a metallic layer of from about 0.5 mu to about 75,000 mu thickness. To protect certain areas of the deposited metal film from chemical attack by acidic solutions there is usually coated over the entire metal film a resist layer, e.g. a photoresist which is sensitive to exposure to light or any other resist material capable of receiving a latent image imparted by a source of energy, such as light, irradiation, electron beam, X-rays, sonar, or heat and capable of being converted from its original form to another such that only exposed areas (positive resist) or unexposed areas (negative resist) can be removed by a developer after exposure to the energy source. A photoresist can be either positive working or negative working and forms an image impressionable layer over the metal film which, in those areas exposed to UV light through a masking device carrying a master pattern, is so modified that a complimentary or corresponding image is registered thereon. The desired areas of the resist are then selectively removed by a developer in which the modified or unmodified portions are soluble but in which the alternate portions are insoluble.
A photoresist is positive working when it reproduces the image, or a reflex copy, of the master pattern by rendering the exposed areas soluble in a given developer in which the unexposed areas are insoluble, and negative working when it reverses the pattern, i.e. by rendering the exposed areas insoluble in a given developer in which the unexposed areas are soluble.
To impart the desired image on the photoresist layer, it is generally covered with a mask or stencil. For example, a patterned shield composed of chromium, a metal or glass on which is deposited in the desired pattern an azo dye, or silver, may be used or any solid opaque material conventionally used for this purpose, apertured in a pattern to provide the desired corresponding or complimentary pattern on the underlaying resist. The resulting assembly is then exposed to the energy source, usually 20 to 500 millijoules/cm.sup.2 exposure to UV light at between 280 and 500 nm wavelength.
In the case of a negative photoresist, e.g. a sensitizer containing irradiation polymerizable or crosslinkable material such as polyvinylcinnamate, a propargyl polymer, an ester of polyvinyl alcohol, a cyclized rubber derivative, an allyl ester prepolymer, etc., the light-exposed portions polymerize or crosslink and thus become insoluble in the developing fluid chosen as the solvent for the process. Conversely, in the case of a positive photoresist which is normally insoluble in the developing fluid, e.g. a diazo oxide or diazide sensitizer-containing polymer such as a novalak resin, an acrylic polymer, copolymer or interpolymer having free carboxyl groups, a polyamic acid condensation product, a styrene-maleic anhydride copolymer, an isoprene polymeric mixture, etc., the light exposed portions are converted to areas of substantial solubility in the selected developing fluid.
After removing the exposed, or unexposed portions of the resist with the developing fluid, the corresponding areas of uncoated metal film are subjected to etching, followed by removing the remaining resist layer from the coated, unetched portions of the film with a suitable solvent, e.g. acetone, carbon tetrachloride, ethyl ketone, chloroform, methylethyl ketone or dimethyl formamide or any other convenient and suitable solvent. The device is thus prepared for implantation of wiring in the etched portions or channels and connected through the areas of metal film remaining on the substrate which act as supports and interconnectors in the circuitry.
It is readily understood that a high degree of accuracy and line resolution with complete and uniform removal of metal from desired portions of the pattern is demanded of the etchant. Numerous etchants and many costly procedures have been proposed to effect complete removal of the metal film in the desired portions while at the same time avoiding undercutting and lifting of the resist-coated metal film at the boundaries delineating the exposed and unexposed areas.
While certain costly etchant solutions of basic, low acid or neutral character have been proposed, these have been found wanting in one respect or another. For example, cerium sulfate or cerium nitrate salts in admixture with sulfuric acid and perchloric acid such as is proposed in British Pat. No. 1,079,607 and German Offenlegungsschrift No. 2,225,105 have succeeded in reducing undercutting to a minor extent; however, they are not significantly improved over the conventional phosphoricnitric-acetic acid etchant solutions and, because of their low acidity, require extended etching time and in many cases provide incomplete or nonuniform removal of exposed metal film. Also the slow etching rate required tends to cause lift or peeling at the edges of the resist coated metal due to mechanical weakening of the coating.
On the other hand, such etchants as the various mixtures of nitric and phosphoric acids which are strongly acidic, show definite effects of undercutting and loss of line delineation, at the boundaries of resist coated metal. When these boundaries are undercut, an indistinct, irregular or jagged line will separate the transparent and opaque areas with an attendant loss in definition of the images photographically registered thereon. Also, the undercut boundaries of the metal film are subject to cracking and crumbling thus causing shorts in the final circuitry. On the other hand, incomplete removal of metal film from desired areas causes bad connections and failure of the circuit.
Another disadvantage of previous fast etching solutions is that they cause formation of hydrogen bubbles which tend to adhere to the metal surface and boundaries of the resist thus preventing uniform or complete etching of the surface which also causes poor line delineation. Nonuniform gloss and spangling effects on metal films of higher thicknesses (25-100 u thickness) which require etching only to a limited depth (e.g. 0.1-0.5 mu depth) is usually attributed to hydrogen gas bubbles adhering to the surface during etching.